Greg Barker, the climate change minister, approved £5 million of taxpayer
funding for a company linked to a controversial adviser in his energy
department.

The minister is facing questions over the influence of the adviser, Miriam Maes, after it emerged she promoted her private clients while working for him as an official consultant.

Internal emails show Mr Barker "approved" an allocation of £5 million to a non-profit company called Salix, which was a partner of Ms Maes's environmental consultancy, Foresee. She was copied into emails relating to the contract.

Ms Maes says there is no commercial relationship between Foresee and Salix but they are part of the same "network".

Critics also claim Mr Barker gave “preferential” treatment to one of Ms Maes's clients, Air Products, while the energy company was trying to win approval for a new energy from waste plant in Teesside.

It emerged last night that Air Products “provided the minister with a hydrogen-powered car to travel between meetings” during an official business trip to the US.

The minister also agreed to meet the company and even intervened on its behalf with the Environment Agency, which was in charge of granting its pollution permits.

Ms Maes asked Mr Barker to use his influence to help Air Products put across its case to the regulator, according to documents released under freedom of information laws.

During a meeting with the Environment Agency, Mr Barker raised the company’s worries that the regulator did not “adequately understand” the benefits of the company's energy-from-waste technology.

On two occasions, in late March and October 2011, Mr Barker’s encounters with Air Products were not disclosed on his official register of meetings. One meeting in early March was disclosed.

Labour MPs last night urged David Cameron to order an investigation. The Prime Minister said his Cabinet Secretary has looked into the matter and found nothing wrong.

Caroline Flint, the shadow energy secretary, said a trail of emails “suggests that Greg Barker personally intervened to secure Miriam Maes’s job” after she worked as a political adviser to him while he was in opposition.

“There can be no cover-ups,” she said. “Greg Barker must now come clean about exactly what involvement he had with the decision to hire Miriam Maes.”

Ms Maes yesterday said she had disclosed her commercial relationships through her company, Foresee, to the department at all times.

“I never furthered any interest of one of my Foresee clients or of my own business and have been totally professional in my support to DECC and the Minister, providing independent expert advice on energy and climate change matters,” she said.

“It was totally justified for Air Products to meet with DECC because of their major energy from waste investment into the UK and due process with the Department was followed at all times.”

A spokesman for the DECC said officials inadvertently failed to disclose Mr Barker’s meeting with Air Products in March 2011. He added that it was legitimate for Mr Barker to raise the concerns of Air Products.

“The Minister raised this concern during a routine meeting with the Environment Agency. It is part of his and the Department’s work to pursue issues raised by industry.”